<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<div id="app">
    {{date}}
</div>
<script src="https://unpkg.com/vue/dist/vue.min.js"></script>
<script>
    var app = new Vue({
        el: '#app',
        data: {
            date: new Date()
        },
        mounted: function (){
            var _this=this;
            this.timer = setInterval(function (){
                _this.date = new Date();
            },1000);
        },
        beforeDestroy: function (){
            if(this.timer){
                clearInterval(this.timer);
            }
        }
    })
</script>

</body>
</html>